1. Field of the Invention
This invention relates to the field of subsurface exploration and, more particularly, to techniques for determining subsurface parameters and well placement. The invention has general application to the well logging art, but the invention is particularly useful in logging while drilling (LWD), measurement-while-drilling (MWD), and directional drilling (geosteering) applications.
2. Background Art
Electromagnetic (EM) logging tools have been employed in the field of subsurface exploration for many years. These logging tools or instruments each have an elongated support equipped with antennas that are operable as sources (transmitters) or sensors (receivers). The antennas on these tools are generally formed as loops or coils of conductive wires. In operation, a transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (“mud”) and into the surrounding formation or formations. (As used herein, “formation” may refer to a single layer or may include multiple layers.) The emitted energy interacts with the borehole and formation to produce signals that are detected and measured by one or more receiver antennas. The detected signals reflect the interactions with the mud and the formation. The measurements are also affected by mud filtrate invasion that changes the properties of the rock near the wellbore. By processing the detected signal data, a log or profile of the formation and/or borehole properties is determined.
The processing of the measured subsurface parameters is done through a process known as an inversion technique. Inversion processing generally includes making an initial estimate or model of the geometry and properties of the earth formations surrounding the well logging instrument. The initial model parameters may be derived in various ways known in the art. An expected logging instrument response is calculated based on the initial model. The calculated response is then compared with the measured response of the logging instrument. Differences between the calculated response and the measured response are used to adjust the parameters of the initial model. The adjusted model is used to again calculate an expected response of the well logging instrument. The expected response for the adjusted model is compared with the measured instrument response, and any difference between them is used to again adjust the model. This process is generally repeated until the differences between the expected response and the measured response fall below a pre-selected threshold. U.S. Pat. No. 6,594,584 describes modern inversion techniques and is incorporated herein by reference in its entirety.
Well placement in real-time using resistivity measurements has been used by the industry since the availability of LWD and MWD tools. This application is commonly known as geosteering. In geosteering, estimation of the borehole position in real-time with respect to known geological markers is performed through correlation of resistivity log features. Because of the typically close placement of the resistivity sensors of a LWD tool along the drill collar relative to the transmitter, only limited radial sensitivity is attained, thereby limiting the extent of the formation geological model knowledge and estimation. With the introduction of sensors having transmitter-receiver distances in the tens of meters can a deeper radial sensitivity can be obtained.
Schlumberger's LWD Ultra Deep Resistivity or UDR™ induction tool, with large transmitter-receiver spacings in the tens of meters, has been successfully tested. One application of the tool has been to determine the location of an oil-water contact (OWC) 7-11 m away from the well path. U.S. Pat. No. 6,188,222, titled “Method and Apparatus for Measuring Resistivity of an Earth Formation” and issued to Seydoux et al., and U.S. Pat. No. 7,093,672, titled “Systems for Deep Resistivity While Drilling for Proactive Geosteering” by Seydoux et al., provide further descriptions of such tools and uses thereof. The '222 patent and the '672 patent are assigned to the assignee of the present invention and are incorporated by reference in their entireties.
The LWD ultra deep resistivity basic tool configuration comprises two independent drilling subs (one transmitter and one receiver) that are placed in a BHA among other drilling tools to allow large transmitter-receiver spacing. The basic measurements obtained with this tool consist of induction amplitudes at various frequencies, in order to allow detection of various formation layer boundaries with resistivity contrasts having a wide range of resistivities. The measurements are used to invert for an optimum parameterized formation model that gives the best fit between actual tool measurements and the expected measurements for the tool in such a formation model.
FIG. 1 shows an example of an MWD tool in use. In the configuration of FIG. 1, a drill string 10 generally includes kelly 8, lengths of drill pipe 11, and drill collars 12, as shown suspended in a borehole 13 that is drilled through an earth formation 9. A drill bit 14 at the lower end of the drill string is rotated by the drive shaft 15 connected to the drilling motor assembly 16. This motor is powered by drilling mud circulated down through the bore of the drill string 10 and back up to the surface via the borehole annulus 13a. The motor assembly 16 includes a power section (rotor/stator or turbine) that drives the drill bit and a bent housing 17 that establishes a small bend angle at its bend point which causes the borehole 13 to curve in the plane of the bend angle and gradually establish a new borehole inclination. The bent housing can be a fixed angle device, or it can be a surface adjustable assembly. The bent housing also can be a downhole adjustable assembly as disclosed in U.S. Pat. No. 5,117,927, which is incorporated herein by reference. Alternately, the motor assembly 16 can include a straight housing and can be used in association with a bent sub well known in the art and located in the drill string above the motor assembly 16 to provide the bend angle.
Above the motor assembly 16 in this drill string is a conventional MWD tool 18, which has sensors that measure various downhole parameters. Drilling, drill bit and earth formation parameters are the types of parameters measured by the MWD system. Drilling parameters include the direction and inclination of the BHA. Drill bit parameters include measurements such as weight on bit (WOB), torque on bit and drive shaft speed. Formation parameters include measurements such as natural gamma ray emission, resistivity of the formations, and other parameters that characterize the formation. Measurement signals, representative of these downhole parameters and characteristics, taken by the MWD system are transmitted to the surface by transmitters in real time or recorded in memory for use when the BHA is brought back to the surface.
Although the prior art deep-reading resistivity tools (such as UDR) proved to be invaluable in geosteering applications, there remains a need for further improved deep-reading resistivity tools that can be used in geosteering and/or other applications.